A high-brightness, power-saving display nowadays is often emerged in many occasions for displaying significant messages, for example, the scoreboard in a large stadium, electronic board in a public place, road sign on a freeway, and so on. Typically, a display is a combination of a great deal of light emitting cells. Currently, the light emitting cell for constituting a display falls roughly into five categories: Incandescent light bulb, small cathode ray tube, high voltage vacuum fluorescent display, small fluorescent lamp, and light-emitting diode.
The incandescent light bulb utilizes the fundamentals of heating the filament to emit light. Because the temperature of the filament which is made of tungsten has to be kept around 900° C. to 1500° C. while the incandescent light bulb is illuminating, the display that is constituted by incandescent light bulbs is very power-consuming and thus the energy efficiency is very low. In addition, because the incandescent light bulb can only emits yellowish white light, it will be quite difficult to be used to constitute a color display.
With respect to the cathode ray tube (CRT), the CRT utilizes electron beam to ram against the phosphor, therefore the luminescent efficiency of the CRT is very high. Theoretically, the energy efficiency of the CRT should be very high. Nevertheless, the electrons in a CRT are produced by heating a hot cathode formed by coating an oxide that is easy to release electrons, e.g. barium oxide, with the surface of metal. While the hot cathode is heating, the oxide is capable of releasing hot electrons. Because the electron gun that is used to produce electrons is a point electron source, the temperature and current of the electron gun have to be boosted in order to obtain a higher electron density. Thus for a light emitting cell requiring to possess high-brightness, the life of the electron gun will inevitably be reduced, and the power consumption will be increased accordingly. On the other hand, because the size of CRT is quite huge, it is not suitable for constituting a high-accuracy display. Moreover, the CRT display is very power-consuming. For example, the power consumption of a 25 m×40 m CRT display is rated at 2000 KW. Though the power consumption of small CRT is only ten percent of that of the incandescent light bulb, the point electron source will result in a low luminescent efficiency.
The high voltage vacuum fluorescent display (HVVFD) is similar to the CRT except that the point electron source is replaced with a line electron source. The line electron source is formed by coating an oxide that is easy to release electrons with a tungsten wire. Because the line electron source can emit numerous electrons to ram against the phosphor, the disadvantage of high power-consumption of the CRT display can be suppressed significantly. Besides, the HVVFD can integrate three original colors—red, green, and blue in a single cell, it is more suitable than CRT for constituting a color display with high resolution.
Nonetheless, though the HVVFD is much better than the CRT, the structure of HVVFD is quite complicated and it is uneasy to be manufactured. Moreover, it will consume a large quantity of power as heating the tungsten filament. For example, the power consumption of a display that is constituted by HVVFD with the size of 25 m×40 m is rated around 1000 kW.
The small fluorescent lamp that utilizes ultraviolet rays to excite the phosphor can also be used to constitute a display. Unfortunately, the colors of the fluorescent lamp today are quite few, and its size is difficult to be dropped below 1 line/mm. Accordingly, it is somewhat difficult to be used to constitute an accurate display.
Light-emitting diode (LED) has been widely employed on a large display today. Though the red, green, and blue LED have been developed thus far, the high-brightness red and blue LEDs are uneasy to be manufactured, and the luminescent efficiency of LED is not comparable to that of the fluorescent lamp. In addition to the disadvantage of low luminescent efficiency, the LED has a serious view angle problem and thus it will not be suitable to be used to constitute a large display.
To conclude, the conventional light emitting cell has the following disadvantages: (a) Low luminescent efficiency, (b) High energy consumption, and (c) Low resolution.
After analyzing the light emitting cells today, it can be found that the luminescent efficiency by using the electrons to ram against the phosphor is superior than that by using other light emitting techniques. Consequently, the small CRT has a better luminescent efficiency than incandescent light bulb, light-emitting diode, and so forth. However, the approach of producing electrons by heating is the major contribution to the power consumption in small CRT and HVVFD. If one is desired to reduce the power consumption, a cold cathode will be the best choice for producing electrons in a light emitting cell.
In 1995, Rinzler first discovered that a carbon nanotube, which is composed of carbon material, can release electrons in “A simple and robust electron beam source from carbon nanotubes” by Philips G. Collins and A. Zettl, Appl., Phys. Lett, 69(13), pp. 1969–1971, 1996. In 1997, Wang et al. discovered that carbon nanotube can release numerous electrons at a low electric field, such as 0.8 V/μm, in “Field emission from nanotube bundle emitters at low fields” by Q. H. Wang, T. D. Corrigan, J. Y. Dai, R. P. H. Chang, and A. R. Krauss, Appl., Phys. Lett, 70(24), pp. 3308–3310, 1997. Consequently, a high-brightness, power-saving, and high-accuracy light emitting cell can be brought out by combining a carbon nanotube at a low electric field and phosphor. The light emitting cell brought out thereby can be used to constitute a monochrome or a color display for displaying static texts and/or dynamic message picture, on an electronic board.
Please refer to the FIG. 1(a), it shows a well-known fiber (usually substituted by carbon nanotube) emission display. The display has a substrate 015 and a panel 010 opposite to each other. A collector electrode 017 is disposed on the panel 010. A light-emitting material 011 is coated on the collector electrode 017. A plurality of row metal lines 013 are disposed on the substrate 015 and plural carbon nanotubes 014 are grown on the row metal line 013 for being a electron source. Plural insulating layers 016 are disposed on the row metal lines 013 and a column metal line 012 is disposed on each of the insulating layer 016. Therefore, the conventional carbon nanotube field emission display is completely.
Please refer to the FIG. 1(b), it is an upper view to show the structure of the column metal lines 012, row metal lines 013 and carbon nanotube 014. It shows that four column metal lines 012a, 012b, 012c, and 012d and three row metal lines 013a, 013b, and 013c are constructed as a network. In operation, please refer to the FIG. 1(a), the column metal lines 012 are biased positively with respect to the row metal lines 013 for extracting the electrons e from the carbon nanotubes 014. The collector electrode 017 is biased by a bias 017′ for further extracting the electrons e to ram against the light-emitting material 011 to emit light. While the column metal line 012 is not biased, the electrons e will not escape from the carbon nanotube 014. Because the column metal lines 012 and the row metal lines 013 are constructed as a network, it is easy to control the address of the electrons e ramming against on the light-emitting material 011. For addressing electrons e on a specific address 018, the column metal lines 012b and 012c should be biased for extracting the electrons e escaping from the carbon nanotubes 014. Therefore, only the area 018′ of the light-emitting material 011, which is over the address 018, can be rammed against by the electrons e to emit light and addressing work is achieved. However, when the prior art carbon nanotube field emission display operates, the address 018 needs not to release the electrons all the time. While the area 018′ needs not to emit light, the column metal lines 012b and 012c will not be biased and the voltage potential thereof will be zero, which means the address 018 will not emit electrons e. After a while when the area 018′ needs to emit light, the column metal lines 012b and 012c will be biased again for extracting the electrons e to escape from the carbon nanotubes 014. Therefore, the column metal lines 012b and 012c will turn on/off many times. However, it is harmful to frequently turn on/off a high voltage unit like the prior art carbon nanotube field emission display, since the frequent power on/off operation will shorten the lifetime of an electric unit. In addition, it is very difficult to drive the device at such high voltage, say 150 to 200 volt. Therefore, the problem of short-lifetime of the prior art carbon nanotube field emission display needs to be solved.
It is therefore the applicant tries to develop a light emitting cell by using a carbon nanotube as an electron source for producing electrons to ram against the light-emitting material (phosphor) for light emission. In the present invention, the carbon nanotubes can continuously release electrons because of the extraction of the extracting electrode. The gate can address the electron by controlling whether the electrons ram against the phosphor or not. Therefore, both the electrode of carbon nanotube and the extracting electrode are not intermittently charged and discharged, so the lifetime of the carbon nanotube field emission display, as the light emitting cell of the present invention, can become longer.